Bandstop or “notch” filters are commonly employed in the CATV industry to block transmission of signals in a specified frequency range. For example, certain channels may be designated as premium channels, requiring payment of a fee from the subscriber in order to receive the signals carrying information representing such channels. If the service is not ordered, i.e., the fee is not paid, an appropriate filter is installed in the cable line coming into the non-paying premises. This is but one of the more traditional uses of bandstop filters, i.e., as a so-called trap.
It is desirable that bandstop filters be as compact as possible commensurate with quality performance of their intended function. It is also desirable, of course, that the filters be as inexpensive as possible, again while maintaining high performance criteria. Passband insertion and return loss is of critical importance in such filters, particularly in the digital signal market, which is steadily growing in significance.
The traditional design of a dual bandstop filter consists of multiple notch filters in series to remove two segments, the stopbands, while preserving the lower, middle, and upper passband signals. A drawback to the traditional design is that the second resonance of the primary notch interferes with the frequencies in the upper passband signal. An additional drawback is that increasing the sharpness of the cutoff by adding more sections increases the insertion loss, as well as increasing the number of circuit components.